1. Field of the Invention
The present invention relates to a film bulk acoustic resonator, a filter, and a fabrication method thereof. Specifically, the present invention relates to a film bulk acoustic resonator including an acoustic reflector portion, a filter and a fabrication method thereof.
2. Description of the Prior Art
As recent mobile phone terminals have been produced in multi-band configurations, multiplexers and interstage filters have been required to have lower insertion loss and steeper attenuation characteristics than were previously required. A film bulk acoustic resonator (FBAR) is a bulk acoustic resonator utilizing the resonance of an elastic wave along the thickness of a piezoelectric film. A FBAR filter which is comprised of the FBARs connected in a ladder configuration has received attention as a filter capable of achieving low-loss and steep attenuation characteristics.
The film bulk acoustic resonator includes a resonator portion. The resonator portion is composed of a piezoelectric film, an upper electrode formed on an upper surface of the piezoelectric film, and a lower electrode formed on a lower surface of the piezoelectric film. In order to confine an acoustic wave generated by excitation of the piezoelectric film to the resonator portion, it is required to further provide a cavity, an acoustic reflector portion, or the like over and under the resonator portion. The acoustic reflector portion is composed of low acoustic impedance layers and high acoustic impedance layers which are alternately stacked on a substrate. The acoustic impedance of the high acoustic impedance layers is higher than that of the low acoustic impedance layers.
Since resonance characteristics of the film bulk acoustic resonator are influenced by crystallinity of a piezoelectric material, a piezoelectric film having good crystallinity is indispensable in order to realize an excellent filter characteristic. Moreover, it is generally known that in order to obtain the piezoelectric film having the good crystallinity, upper surfaces of the lower electrode formed under the piezoelectric film and also of the acoustic reflector portion formed under the lower electrode have to be smoothed.
When the low acoustic impedance layers and the high acoustic impedance layers are alternately formed on the substrate by general chemical vapor deposition (CVD), sputtering, or the like to form the acoustic reflector portion, bumps and dips on a surface of each of the layers are made greater due to a layer-stacking configuration. Therefore, it is very difficult to smooth a surface of the acoustic reflector portion.
For example, if a film forming condition is optimized when each of the layers included in the acoustic reflector portion is formed by CVD or sputtering, the root-mean-square roughness (Rms) of a surface of one layer can be suppressed to about several nanometers. However, if a plurality of layers is formed, the surface roughness is accumulated. As a result, the Rms of the surface of the configured acoustic reflector portion has a value of about several tens of nanometers.
To cope with this problem, it is attempted that after the acoustic reflector portion is formed, the surface of the acoustic reflector portion is smoothed by chemical mechanical polishing (CMP), or that an acoustic reflector portion is formed by bias sputtering to smooth a surface of the acoustic reflector portion (for example, see Japanese Laid-Open Patent Publication No. 2005-136761).
However, the above-mentioned conventional film bulk acoustic resonator has such a problem that a fabrication method thereof is complicated. Bias sputtering requires special equipment, and setting a sputtering condition is also troublesome. Moreover, there is a problem that when the upper surface of the acoustic reflector portion is smoothed by CMP, steps are complicated. Especially, since the thickness of each layer included in the acoustic reflector portion has to be accurately adjusted to the wavelength of the acoustic wave which is generated by excitation of the piezoelectric film in the resonator portion, it is also necessary to monitor the polishing amount as well as the smoothness.